1. Field of the Invention
The present invention is concerned generally with mattress constructions and more specifically with mattresses forming water beds.
2. Description of the Prior Art
In its simpliest form, a water bed includes a bag or bladder having the generally flat configuration of a mattress and defining a fluid tight cavity. A valve extending into the bag permits filling of the cavity with a fluid such as water. The bags are typically constructed of a flexible material such as vinyl which can be heat sealed to itself to provide the bag with a desired configuration. Mattresses of this type, such as that disclosed by Hall in U.S. Pat. No. 3,585,356, are well known to provide an excellent surface upon which a person can lie. The water in the bag provides for an even distribution of the supporting force so that the pressure on the body of the person is substantially constant over all areas of contact. In the absence of high pressure areas of contact, the water bed has excellant therapeutic characteristics which enable a person to lie in one position for an extended period of time.
A problem common to all water mattresses is associated with the significant fluid pressures which are developed within the bag due to the large quantity of water in the cavity. These pressures have a particularly adverse affect on the seams which are formed when the various pieces of the material are bonded together.
It is known that lap seams are generally more desirable than butt seams in opposing these significant pressures. A lap seam is placed in shear stress by such pressures while a butt seam is placed in tensile stress by such pressures. Since the shear stress of a seam typically exceeds the tensile stress of a seam, the lap seam construction is generally preferred for water beds. A water mattress illustrating lap seam construction is disclosed by Kuss in U.S. Pat. No. 3,753,823.
In order to form a lap seam, a radio frequency heat sealer is used in conjunction with a die rod which is pressed into contact with overlapping portions of the material to be joined. Then radio frequency waves are transmitted by the heat sealer through the die rod to melt the overlapping portions of the material and effect a seal therebetween.
A lap seam construction for a bag is more difficult. In such a construction, a heat sink rod or buffer must be provided interiorly of the bag with the die rod pressed against the overlapping portions and against the heat sink rod. The heat sink rod ensures that only the material between the heat sink rod and the die rod is melted by the radio frequency waves. Other portions of the bag on which the heat sink rod might rest are not melted.
Of course, once the heat sink rod or buffer is disposed within the cavity, it must remain in the cavity or otherwise be removed from the cavity. In the Kuss U.S. Pat. No. 3,753,823, access holes are provided through which heat sink rods can be disposed to provide for the lap seam construction. When the final seam is made to seal the access holes, a buffer sheet is used which remains within the sealed bag.
Water mattresses consisting of only a single bag or bladder are typically supported within a rigid frame having side members which provide lateral support for the water filled bag. In other types of construction, a second cavity has been provided to form a loop around the sides of the water filled cavity. This second cavity has been filled with air to provide an air frame for the water mattress. With this configuration, no rigid side support members need be provided for the water mattress. U.S. Pat. No. 3,787,907, issued to Pennington, is representative of this air frame construction. A patent issued to Penn, U.S. Pat. No. 3,778,852, also shows an air frame construction. Other patents including pneumatic side walls include those issued to Whitney, U.S. Pat. No. 3,842,455, and Shields, U.S. Pat. No. 3,766,579.
Although the provision of a pneumatic chamber surrounding a water mattress has significant advantages, this construction in the past has called for the provision of many separate pieces of material and the formation of many bonds in order to provide the desired configuration. Of course, the greater number of bonds the greater is the possibility that a seal will be formed which is too weak to withstand the significant pressures associated with the water in the mattress. In order to speed the construction of this type of mattress, many of these seals have been formed with the less desirable butt seam construction. For example, vertical butt seams provided in the corners of some of the mattresses have been particularly susceptible to rupture.